11.5 compression, Iron Head, 91 octane

[ClassAct]

Get your tune up correct and it will be fine.

[Stan Weiss]

Is the an IVC of 74* at .050"?

Could be at the seat-to-seat

Stan

PS - An example - 292 intake @ 108 ICL 38/74

ab-cpg-scr-dcr-ivc.gif

[dfarr67]

If you crutch it for the street- are you really going to be happy in a competitive environment? Can you run propane for the street or use a different carb and run EXX ?

[Topsp33d]

IMG_0410.jpeg

These are the n351 heads on a 4.100 stroke Windsor. I bought the heads used and never poured the chambers until recently, after I invested in the offset shaft rockers and custom header etc. They were supposed to be 64cc but apparently they got cleaned up a little too much. I would consider swapping heads for something with a bigger chamber. I would have to find something with the same flange I guess

The short block is still being assembled but the pistons have no room for porting, they are dished and short. I may be able to order a custom set if the wife doesn’t find out

I want to take a look at engine analyzers to calculate the knock, seems cheapest option at the moment.

[Topsp33d]

Here is the ls7 chamber

IMG_0397.jpeg

Here’s the piston, I thought about tracing the dish onto the chamber to open up. I know that could potentially screw the whole head, I’m already doubting this idea haha

image.jpg

Something like this maybe

IMG_0411.jpeg

[mt-engines]

IMG_0410.jpeg
These are the n351 heads on a 4.100 stroke Windsor. I bought the heads used and never poured the chambers until recently, after I invested in the offset shaft rockers and custom header etc. They were supposed to be 64cc but apparently they got cleaned up a little too much. I would consider swapping heads for something with a bigger chamber. I would have to find something with the same flange I guess

The short block is still being assembled but the pistons have no room for porting, they are dished and short. I may be able to order a custom set if the wife doesn’t find out

I want to take a look at engine analyzers to calculate the knock, seems cheapest option at the moment.

Maybe an illusion, but that valve job doesn't look good to me. But either way, just blending the top cut into the chambers should help you out. I could see 2 cc just in blending whats there.

But id probably use different seat profiles, you'd have to sink the valve job a bit, thats another cc.

[HQM383]

If all it took was a later closing intake valve a Pro Stock racing engine would run on 87 octane gas.
It doesn't. Your engine is going to want 100 R+M/2 octane gas.
For street use on a pump gas retard the timing as required to avoid detonation.
4-6-8 deg as needed.
Water/methanol injection does wonders too.

Does this use the formula:

Static compression ratio + cylinder head material = fuel octane rating?

No formula.. 45 years experience.

With your experience if he changed to aluminum heads what would the Octane requirements be then?

[cab0154]

Some of the Japanese stuff make over 3hp per ci on 93 at close to 13:1 with port injection/carburetion with aluminum turning a ton of rpm. I would try to make it work with iron by running a later IVC and keeping the coolant temp in check. with a stock cam, iron heads, and boost on the stock shortblock of my 93 mustang gt, a turbo technology street kit on 93 octane pump gas back in the 90s with nothing more than a MSD BTM and an FMU. and that was in the summer in TX, with the coolant temp at 170*.

[mt-engines]

Does this use the formula:

Static compression ratio + cylinder head material = fuel octane rating?

No formula.. 45 years experience.

With your experience if he changed to aluminum heads what would the Octane requirements be then?

Just ignore him.. his experience is 45 years of regurgitated hearsay.

[HQM383]

To get a better understanding of this situation with stick shift and maybe help the OP.

Engine is not yet assembled. What is the better choice in this situation:

Leave compression as is and choose camshaft IVC that will help prevent combustion issues but likely shift torque curve up in rpm but make more top end hp

Or

Decide what fuel to use/available/cost effective. Choose a cam that compliments vehicle weight and gearing for torque curve rpm that mat make a few less top end hp and adjust compression ratio to work with above. (I believe what mt engines was getting at in his first post)

Is that a reasonable way to look at it?

[Tuner]

IMG_0410.jpeg
These are the n351 heads on a 4.100 stroke Windsor. I bought the heads used and never poured the chambers until recently, after I invested in the offset shaft rockers and custom header etc. They were supposed to be 64cc but apparently they got cleaned up a little too much. I would consider swapping heads for something with a bigger chamber. I would have to find something with the same flange I guess

The short block is still being assembled but the pistons have no room for porting, they are dished and short. I may be able to order a custom set if the wife doesn’t find out

I want to take a look at engine analyzers to calculate the knock, seems cheapest option at the moment.

Maybe an illusion, but that valve job doesn't look good to me. But either way, just blending the top cut into the chambers should help you out. I could see 2 cc just in blending whats there.

But id probably use different seat profiles, you'd have to sink the valve job a bit, thats another cc.

^^ This^^ "blending the top cut" indeed. This looks like a victim of some brutal valve grinds by someone who didn't think sinking seats is a bad idea. A proper radius from the top of the seat to the chamber wall to eliminate all those steps and lips will improve flow and give you some chamber volume.

A friend built a 11.5/1 392 Windsor using a pair of (?? budget chinese ?? I forget) aluminum heads with chambers looking about like the N head. I recall the cam was mid 220s @.050. After the owner installed it with an "all in at 2500" curve and it knocked like it was crushing rocks we did some road testing with a timing light and determined the curve needed to be similar to 20 at 2000 with a slope to 26 at 6000. The vacuum advance needed to be fully retarded by 10" to avoid knock at part throttle. With the proper distributor set up it ran great with excellent torque at any RPM and pulled like a tractor even as low as 1500 with the AOD in overdrive and the converter locked, however it was in a lightweight car, a '66 Mustang.

The point is spark advance management over the entire RPM and load range is key to best performance with any engine, much more important than is generally realized, and particularly if the engine is skating the edge of maximum compression ratio for the fuel used.

Lowering compression ratio to force an engine to accommodate more spark advance because "more spark advance is better" is insane.

Recent improvements in fast burn heads are happy with higher compression ratios than prefiouslywere considered "too much" simply because fast burn requires less advance.

On an aside ... last weekend I watched a N head Ford totally dominate the 35 whatever head Chevrolets behind it for 125 laps on a half-mile asphalt oval. When it was down to lap 120 after leading 110 he ran 4 laps a half-second faster than all the previous. Some days it isn't just to win, it is to humiliate the losers so their women are ashamed to be seen with them.

[mt-engines]

To get a better understanding of this situation with stick shift and maybe help the OP.

Engine is not yet assembled. What is the better choice in this situation:

Leave compression as is and choose camshaft IVC that will help prevent combustion issues but likely shift torque curve up in rpm but make more top end hp

Or

Decide what fuel to use/available/cost effective. Choose a cam that compliments vehicle weight and gearing for torque curve rpm that mat make a few less top end hp and adjust compression ratio to work with above.

Is that a reasonable way to look at it?

Timing curve is a big factor. If he could run a grid and use engine vacuum to pull timing, that would make it better.
MPFI would help it a ton!

The key is to have a homogenized air fuel mixture and to keep it suspended into the cylinder.

Thats why I suggest he work the chambers. He will gain some CC and drop the compression a bit. But the real help would be in mixture. Id want some nice sharp angles and a steeper top cut.

[F-BIRD'88]

No formula.. 45 years experience.

With your experience if he changed to aluminum heads what would the Octane requirements be then?

Just ignore him.. his experience is 45 years of regurgitated hearsay.

Hardly, fool.

[Tuner]

There is no answer forthcoming for that "if aluminum heads etc ..." question?

[Topsp33d]

I have a problem with the hotrod swap meets where I have to buy something if it’s Windsor or mustang, so I have carbs and distributors and cams and intakes. Using the big cam the best case I can see making this work is to blend the chambers to the seats and maybe a touch more, trying to find the best curve, running a low thermostat, sealed cold air to cowl vent, etc

MPFI is totally an option, I have another thread that I started regarding that here viewtopic.php?t=68277 where I stated I have a nice efi setup but wanted to go with a carb for a more mechanical experience.

Another option is to skip the squish completely with a cometic .120 gasket which places me at a safe cr for the more street friendly cams.

I plan to unshroud and touch up the chambers regardless of which direction we go. I think they bent a valve in cylinder 1, I can see some dings, so they cleaned it up by milling too much and then compensated by sending the valve job too deep? Should I do another valve job or just roll the radius out each way? What do sharper angles do for this situation